The Fundamental Basis for Cyclopolymerization. X. A Systematic Study of the Cyclopolymerization of Methacrylic Anhydride

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Abstract
The study of methacrylic anhydride has shown that the intramolecular propagation step has a higher energy of activation (2.6 ± 0.3 kcal/mole) than the intermolecular step. Steric factors have, however, been found to favor cyclization (A11/Ac = 0.0039 liter/mole). It has been concluded that the conditions which favor cyclization in methacrylic anhydride are high temperature, low monomer concentration, poor solvent system, and high conversion. The increase in cyclization caused by heterogeneous conditions, such as poor solvent or high conversion, appears to be due to tight coiling of the polymer chain and slow diffusion, thereby decreasing the value of k11 with respect to kc. Although methacrylic anhydride does cross-link to some extent, the number of cross-links was lower than expected due to anhydride interchange of pendent units. Also, even though some cross-links were present, the polymer could be dissolved due to interchange of the intermolecular cross-links to intramolecular anhydride units. The bathochromic shift observed for methacrylic anhydride, when its UV spectrum was compared with those for the methacrylate esters, was found to result from added electronic stabilization of the α, β-unsaturated system through resonance of the nonbonded electrons on the acyl oxygen with the carbonyl unit attached to it. In a stereochemical configuration study on methacrylic anhydride, no solvent effect could be found. The influence of increasing temperature and conversion was found to be small, but appeared to increase the heterotacticity and decrease the syndiotacticity by equivalent amounts. Noncyclic anhydride units resulted in increased syndiotactic character. The tacticities observed were explained on the grounds of a random cyclic propagation step and a stereospecific intermolecular step. The results of the model study on methacrylic anhydride have been combined with the literature pertinent to the mechanism and stereochemistry of cyclopolymerization in order to develop a general understanding of these areas. After thorough consideration, it seems that the driving-force for cyclization in cyclopolymerizations is due to: 1) statistical probability, 2) thermodynamic stability, and 3) steric and entropy effects. In many dienes, nonconjugated excited state interactions are also very important. The driving force for the higher degrees of polymerization, observed in cyclopolymerization for difficultly polymerizable systems, may be due to steric factors. Stereochemical studies have shown that the high degrees of order which are observed in cyclopolymerization are due to the cyclic units that are formed. It also appears that free radical cyclopolymerizations yield the kinetically controlled product while the alkyl-metal coordination catalysts give the thermodynamic product.